Soy peptides enhance yeast cell growth at low temperatures

A comprehensive review and comparison of L-tryptophan biosynthesis in Saccharomyces cerevisiae and Escherichia coli

L-tryptophan and its derivatives are widely used in the chemical, pharmaceutical, food, and feed industries. Microbial fermentation is the most commonly used method to produce L-tryptophan, which calls for an effective cell factory. The mechanism of L-tryptophan biosynthesis in Escherichia coli, the widely used producer of L-tryptophan, is well understood. Saccharomyces cerevisiae also plays a significant role in the industrial production of biochemicals. Because of its robustness and safety, S. cerevisiae is favored for producing pharmaceuticals and food-grade biochemicals. However, the biosynthesis of L-tryptophan in S. cerevisiae has been rarely summarized. The synthetic pathways and engineering strategies of L-tryptophan in E. coli and S. cerevisiae have been reviewed and compared in this review. Furthermore, the information presented in this review pertains to the existing understanding of how L-tryptophan affects S. cerevisiae's stress fitness, which could aid in developing a novel plan to produce more resilient industrial yeast and E. coli cell factories.

Improving the productivity of malic acid by alleviating oxidative stress during Aspergillus niger fermentation

BACKGROUND

As an attractive platform chemical, malic acid has been commonly used in the food, feed and pharmaceutical field. Microbial fermentation of biobased sources to produce malic acid has attracted great attention because it is sustainable and environment-friendly. However, most studies mainly focus on improving yield and ignore shortening fermentation time. A long fermentation period means high cost, and hinders the industrial applications of microbial fermentation. Stresses, especially oxidative stress generated during fermentation, inhibit microbial growth and production, and prolong fermentation period. Previous studies have shown that polypeptides could effectively relieve stresses, but the underlying mechanisms were poorly understood.

RESULTS

In this study, polypeptides (especially elastin peptide) addition improves the productivity of malic acid in A. niger, resulting in shortening of fermentation time from 120 to 108 h. Transcriptome and biochemical analyses demonstrated that both antioxidant enzyme-mediated oxidative stress defense system, such as superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX), and nonenzymatic antioxidant system, such as glutathione, were enhanced in the presence of elastin peptide, suggesting elastin peptide relieving oxidative stresses is involved in many pathways. In order to further investigate the relationship between oxidative stress defense and malic acid productivity, we overexpressed three enzymes (Sod1, CAT, Tps1) related to oxidation resistance in A. niger, respectively, and these resulting strains display varying degree of improvement in malic acid productivity. Especially, the strain overexpressing the Sod1 gene achieved a malate titer of 91.85 ± 2.58 g/L in 96 h, corresponding to a productivity of 0.96 g/L/h, which performs better than elastin peptide addition.

CONCLUSIONS

Our investigation provides an excellent reference for alleviating the stress of the fungal fermentation process and improving fermentation efficiency.

Protective effects of peptides on the cell wall structure of yeast under osmotic stress

Three peptides (LL, LML, and LLL) were used to examine their influences on the osmotic stress tolerance and cell wall properties of brewer's yeast. Results suggested that peptide supplementation improved the osmotic stress tolerance of yeast through enhancing the integrity and stability of the cell wall. Transmission electron micrographs showed that the thickness of yeast cell wall was increased by peptide addition under osmotic stress. Additionally, quantitative analysis of cell wall polysaccharide components in the LL and LLL groups revealed that they had 27.34% and 24.41% higher chitin levels, 25.73% and 22.59% higher mannan levels, and 17.86% and 21.35% higher β-1,3-glucan levels, respectively, than the control. Furthermore, peptide supplementation could positively modulate the cell wall integrity pathway and up-regulate the expressions of cell wall remodeling-related genes, including FKS1, FKS2, KRE6, MNN9, and CRH1. Thus, these results demonstrated that peptides improved the osmotic stress tolerance of yeast via remodeling the yeast cell wall and reinforcing the structure of the cell wall. KEY POINTS: • Peptide supplementation improved yeast osmotic stress tolerance via cell wall remodeling. • Peptide supplementation enhanced cell wall thickness and stability under osmotic stress. • Peptide supplementation positively modulated the CWI pathway under osmotic stress.

A modified xanthine oxidase cell model for screening of antihyperuricemic functional compounds

Hyperuricemia is a purine metabolism disorder, with increasing prevalence worldwide. Here, a high throughput cell model for screening of antihyperuricemic compounds was set up. Human kidney cells (HK2 cells) were stimulated with adenosine and the resulting supernatant and lysate were then analyzed using high performance liquid chromatography (HPLC). The results showed that hypoxanthine content was increased in both HK2 cells supernatant and xanthine oxidase (XO)-overexpressing HK2 cells lysate, but no uric acid was detected due to lower endogenous XO content in these cells. Exogenous XO was added to the supernatant, and then used to evaluate the antihyperuricemic activity of Febuxostat and two the previously identified peptides, Pro-Gly-Ala-Cys-Ser-Asn (PGACSN) and Trp-Met-Leu (WML). By adding exogenous XO, this combined-adenosine-XO-induced hyperuricemia model was optimized and established, and the Febuxostat and peptides were confirmed to significantly reduce uric acid production in the HK2 cells supernatant (p < 0.05). Therefore, this cell model could be recommended for screening potential bioactive antihyperuricemic compounds.

Improved osmotic stress tolerance in brewer's yeast induced by wheat gluten peptides

The influences of three wheat gluten peptides (WGP-LL, WGP-LML, and WGP-LLL) on the osmotic stress tolerance and membrane lipid component in brewer's yeast were investigated. The results demonstrated that the growth and survival of yeast under osmotic stress were enhanced by WGP supplementation. The addition of WGP upregulated the expressions of OLE1 (encoded the delta-9 fatty acid desaturase) and ERG1 (encoded squalene epoxidase) genes under osmotic stress. At the same time, WGP addition enhanced palmitoleic acid (C16:1) content, unsaturated fatty acids/saturated fatty acids ratio, and the amount of ergosterol in yeast cells under osmotic stress. Furthermore, yeast cells in WGP-LL and WGP-LLL groups were more resistant to osmotic stress. WGP-LL and WGP-LLL addition caused 25.08% and 27.02% increase in membrane fluidity, 22.36% and 29.54% reduction in membrane permeability, 18.38% and 14.26% rise in membrane integrity in yeast cells, respectively. In addition, scanning electron microscopy analysis revealed that the addition of WGP was capable of maintaining yeast cell morphology and reducing cell membrane damage under osmotic stress. Thus, alteration of membrane lipid component by WGP was an effective approach for increasing the growth and survival of yeast cells under osmotic stress. KEY POINTS: •WGP addition enhanced cell growth and survival of yeast under osmotic stress. •WGP addition increased unsaturated fatty acids and ergosterol contents in yeast. •WGP supplementation improved membrane homeostasis in yeast at osmotic stress.

Wheat Gluten Peptides Enhance Ethanol Stress Tolerance by Regulating the Membrane Lipid Composition in Yeast

Wheat gluten peptides (WGPs), identified as Leu-Leu (LL), Leu-Leu-Leu (LLL), and Leu-Met-Leu (LML), were tested for their impacts on cell growth, membrane lipid composition, and membrane homeostasis of yeast under ethanol stress. The results showed that WGP supplementation could strengthen cell growth and viability and enhance the ethanol stress tolerance of yeast. WGP supplementation increased the expressions of OLE1 and ERG1 and enhanced the levels of oleic acid (C18:1) and ergosterol in yeast cell membranes. Moreover, LLL and LML exhibited a better protective effect for yeast under ethanol stress compared to LL. LLL and LML supplementation led to 20.3 ± 1.5% and 18.9 ± 1.7% enhancement in cell membrane fluidity, 21.8 ± 1.6% and 30.5 ± 1.1% increase in membrane integrity, and 26.3 ± 4.8% and 27.6 ± 4.6% decrease in membrane permeability in yeast under ethanol stress, respectively. The results from scanning electron microscopy (SEM) elucidated that WGP supplementation is favorable for the maintenance of yeast cell morphology under ethanol stress. All of these results revealed that WGP is an efficient enhancer for improving the ethanol stress tolerance of yeast by regulating the membrane lipid composition.

Tartary buckwheat protein hydrolysates enhance the salt tolerance of the soy sauce fermentation yeast Zygosaccharomyces rouxii

Supplementation of protein hydrolysate is an important strategy to improve the salt tolerance of soy sauce aroma-producing yeast. In the present study, Tartary buckwheat protein hydrolysates (BPHs) were prepared and separated by ultrafiltration into LM-1 (<1 kDa) and HM-2 (1-300 kDa) fractions. The supplementation of HM-2 fraction could significantly improve cell growth and fermentation of soy sauce aroma-producing yeast Zygosaccharomyces rouxii As2.180 under high salt (12%, w/w) conditions. However, the LM-1 fraction inhibited strain growth and fermentation. The addition of HM-2 promoted yeast cell accumulation of K⁺, removal of cytosolic Na⁺ and accumulation of glycerol. Furthermore, the HM-2 fraction improved the cell membrane integrity and mitochondrial membrane and decreased intracellular ROS accumulation of the strain. The above results indicated that the supplementation of BPHs with a molecular weight of 1-300 kDa is a potentially effective and feasible strategy for improving the salt tolerance of soy sauce aroma-producing yeast Z. rouxii.

Soybean peptides promote yoghurt fermentation and quality

OBJECTIVES

This research paper was to investigate the influence of soybean peptides addition on viable count of lactic acid bacteria, physicochemical parameters, flavor, and sensory evaluation of yoghurt.

RESULTS

The number of fermenting strains (Streptococcus thermophilus + Lactobacillus delbrueckii subsp. bulgaricus) cells in yoghurt (stored at 4 °C for 19 days) added with 0.2% (w/v) of soybean peptides (808.34 Da) reached 1.4 times higher bacterial number than in the control group. A total of 34 volatile substances were detected in this study, while there were 22 volatiles occurred in the control group yoghurt, 30 volatiles were detected in yoghurt added with 0.2% soybean peptides. There was no significant difference in sensory evaluation (p > 0.05) between the yoghurt with and without soybean peptides.

CONCLUSIONS

In our study, the addition of soybean peptides (0.2%) can be effective both in maintaining the viable bacterial count and yoghurt quality.